I received my Obi202 VoIP ATA yesterday, took it out of the box, tossed aside the counterfeit PAP2T, plugged it all in and fired up the web interface (dial ***030, option 1). I entered the settings for sipgate.co.uk (from their website), but kept getting "403 - No valid Username" whenever I tried to register. Googling didn't seem to help, so I thought I'd share the solution here.
The OBi202 has 3 fields (amongst others) under System Management -> Voice Services -> SPx service, they are AuthUserName, AuthPassword, URI.
I set them to my sipgate 8-digit username, my sipgate password, and then guessed at "sipgate.co.uk" for the URI. I then wasted an hour trawling through the other settings (of which there are many) - none of which made any difference.
The solution? URI isn't a url or domain name, it's just your username, so set it exactly the same as the AuthUserName, and everything will work perfectly.